Supported catalyst activation

Details Supported catalyst activation Supported catalyst activation

2002-01-29

2004-06-22

Parsa, J. (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Liquid phase fischer-tropsch reaction

C518S713000, C518S715000, C518S709000, C518S710000, C502S038000, C502S053000, C502S054000, C502S055000, C502S326000, C502S331000, C208S20800M, C208S106000

Reexamination Certificate

active

06753351

ABSTRACT:

This invention relates to a process for the activation of supported Fischer-Tropsch catalysts that enhances their activity and/or selectivity in the production of higher hydrocarbons from synthesis gas.
BACKGROUND OF THE INVENTION
The conversion of synthesis gas, i.e. carbon monoxide and hydrogen, to higher value products is well known and has been in commercial use for many years. Typical processes include, for example, methanol syntheses, higher alcohol synthesis, hydroformylation and Fischer-Tropsch synthesis. The synthesis gas mixture is contacted with a suitable catalyst typically comprising at least one Group VIII metals. Suitable Fischer-Tropsch catalysts comprise one or more catalytic Group VIII metals, such as iron, cobalt and nickel. For oxygenate synthesis, copper may be included as well.
There exist many variations of the formulation and preparation of catalysts useful for the conversion of synthesis gas. In general, the catalysts are classified into two broad types, unsupported metals, known as Dispersed Active Metals and a larger groups of catalysts metals supported on refractory oxides, such as silica, alumina, titania or mixtures thereof. Such catalysts, whether supported or unsupported may be enhanced by the addition of other metals or metal oxides, known as promoter metals.
Supports for catalyst metals are generally pilled, pelleted, beaded, extruded, spray-dried or sieved materials. There are many methodologies reported in the literature for the preparation of supported catalyst metals. Examples of such techniques include incipient wetness impregnation, slurry impregnation, coprecipitation, and the like. It will be appreciated that high metal loadings are generally obtained by coprecipitation or multiple, i.e. two or three, impregnations, whereas low metal loading catalysts may be prepared utilizing a single impregnation. The catalyst metal content of such catalysts may vary from one to fifty weight percent. Promoter metals or metal oxides may be added during the impregnation steps using soluble salts of the respective metals such as Pt, Pd, Rh, Ru, Os, Ir, Mo, W, Cu, Si, Cr, Ti, Mg, Mn, Zr, Hf, Al, Th and the like.
It will further be appreciated that the choice of a particular metal combination and the amount thereof to be utilized will depend upon the specific application used in the conversion of synthesis gas. When a suitable support has been impregnated with one or more metals as by impregnation to form a catalyst precursor, it may be dried and then calcined in an oxygen-containing environment. The precursor is thereafter activated by reduction at elevated temperature in the presence of a reducing gas, typically containing hydrogen. Optionally, the catalyst is activated by contacting with hydrogen gas in presence of liquid hydrocarbons as disclosed in U.S. Pat. No. 5,292,705.
Regardless of the particular formulation and method of preparation, the method of activation, which may include a pretreatment, impacts the productivity and/or selectivity of the catalyst. Selectivity is generally expressed in terms of the percent of an undesirable substance in the product mix. For example, methane selectivity in a Fischer-Tropsch reaction is the percent of methane formed with the desired higher hydrocarbons. If productivity is often tied to a specific catalyst synthesis method or catalyst activation method, low productivity may also result from problems that may occur during the activation process itself. For example it is well known that the reduction of cobalt containing catalyst under conditions in which high partial pressure of water are obtained, lead to a poor catalyst activation and low catalyst productivity. In some cases, such poorly activated catalysts, which cannot be used at all, are discarded and then treated for metal recovery. It is obviously commercially significant to provide a method for the activation of catalyst that boosts the productivity and selectivity of catalysts, especially poorly activated catalysts, thereby avoiding the significant expense of their disposal.
Typically, metal containing catalysts are activated by treatment at elevated temperatures in presence of a reducing gas, for example a hydrogen-containing gas. In some specific applications such hydrogenation reactions for specialty chemicals, the metal component of the catalyst may be reduced at lower temperature using other reducing reagents such hydrazine or alkyl aluminum to maximize the metal dispersion or catalyst activity. Formation of a reduced catalyst may also be achieved by direct decomposition of metal salts, for example, thermal decomposition of oxalates. Carbon monoxide hydrogenation catalysts are commonly activated by means of high temperature reduction in presence of a hydrogen-containing gas. Typical procedures ensure a low partial pressure of water during the reduction by controlling the rate of reduction of the metal oxides. There are known pretreatment methods described in the literature.
U.S. Pat. Nos. 4,492,774; 4,595,798; 4,088,671; 4,605,679; and 4,670,414 and EP 0 253 924 disclose a method of activation of cobalt catalysts by means of a reduction/oxidation/reduction (R—O—R) cycle, resulting in an increase in activity for Fischer-Tropsch synthesis. To our knowledge, all the oxidation/reduction or reduction/oxidation/reduction cycles described in the above patents, were effected by treating a dry solid catalyst with an oxygen-containing gas at high temperature, resulting in the formation of the most stable oxide i.e., Co
3
O
4
. Several of these citations stressed the importance of controlling the exothermicity of the oxidation reaction and ensuring a low partial pressure of water during the reduction to avoid sintering of the cobalt particles, which may be detrimental to the activity of the final catalyst.
Khodakov et al. In a paper in Oil & Gas Science and Technology Rev. IFP, 54, 525 (1999) teach that contacting a reduced cobalt catalyst with water, followed by drying and calcining in air results in the formation of smaller cobalt oxide crystallites relative to those that would be formed by decomposition of the initial cobalt salts.
It is generally recognized that the economic worth of a given catalyst is a function of its original cost and its activity. It is apparent from the foregoing discussion that there has been considerable effort going back over many years to improve the economic worth of catalysts, since a process that will effectively increase the activity of a catalyst and/or extend the useful life will significantly improve the worth of that catalyst. Such a process is provided in accordance with the present invention.
SUMMARY OF THE INVENTION
In accordance with the present invention, catalytic activity and/or methane selectivity of supported Fischer-Tropsch metal catalysts or catalyst precursors are enhanced by a process comprising initially reducing with a hydrogen-containing gas at elevated temperatures to cause at least part of the metal therein to be in the metallic state, impregnating under a non-oxidizing atmosphere with a solution of at least one member selected from the group consisting of ammonium salts, alkyl ammonium salts and weak organic acids, optionally in combination with ammonia, oxidizing in the presence of the impregnating solution at low temperatures and reducing with a hydrogen-containing gas at elevated temperatures to activate the catalyst. Optionally, the supported catalyst precursor is calcined in the presence of an oxidant-containing atmosphere prior to activation. The activated catalyst may also be passivated to further enhance its properties.
DETAILED DESCRIPTION OF THE INVENTION
Supported metal catalysts, which correspond essentially to reduced metals formed by one of the recognized techniques discussed above onto a suitable support structure, typically a refractory inorganic oxide, such as titania, silica, silica-alumina, aluminum and the like, are utilized in a broad range of applications such as hydrogenation of hydrocarbons and carbon monoxide. Titania is a preferred support material for the catalyst metal sub

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